In a linear scanning type ultrasonic imaging apparatus, ultrasound is received and transmitted by simultaneously driving the elements of an arrayed transducer group forming an aperture, which are selected by a aperture-selecting switch to form the aperture for a plurality of ultrasonic array transducers. Then, by shifting the aperture successively, the inside of the living body or goods is linear-scanned by an ultrasonic beam. Also, in a sector scanning type ultrasonic imaging apparatus, the inside of the living body or goods is scanned by inclining the ultrasonic beam, not shifting the aperture. In both the linear method and the sector method, a focus point is set up in the living body or the goods, and then a driving pulse is provided to each transducer in a delay-controlled manner so that all the ultrasonic waves transmitted from ultrasonic transducer group in the aperture can arrive at the focus point simultaneously.
For performing the delay control, a transmission delay circuit is provided. Output of the driving pulse generated from a transmitting circuit is delayed, said driving pulse is supplied to each transducer in the selected aperture through the transmission delay circuit, and ultrasonic beam is transmitted.
Reflection echoes from the object are received by a plurality of the ultrasonic transducers in the receiving aperture selected, and said reflection echoes are input to a receiving circuit connected to this plurality of ultrasonic transducers through an transmitting/receiving separation circuit. Said echoes are turned into signals having a good amplified dynamic range, and then, said signals are converted into digital signals by a plurality of analog digital converters. These signals are time-converted so that all received echoes arrive at the same time and then added up and output with a phase adjustment unit, which is comprised of a digital delay unit and an adder circuit. This output is used as receiving beam signals. Logarithm compression, filtering and γ correction are performed on this output by a signal processing unit, and the output is displayed after performing conversion of the data, such as coordinate transformation, or interpolation.
Delay data for delaying transmission and reception of signals described above is calculated by dividing the distance from each transducer to the focus point by propagation velocity of ultrasound in the object, thus deriving a time value. But, the structure of the medium to be examined is not uniform. Propagating velocity of ultrasound within a body is varied, depending for instance on whether the person is obese or muscular. Thus, in the present circumstance, delay data is set up in the apparatus by positing the average velocity of ultrasonic waves propagating in the living body.
When the actual sonic velocity is exceedingly different from posited velocity due to individual differences, a clear image cannot be obtained since ideal focusing is not performed.
As an example of method of estimating ultrasonic propagation velocity in living tissue, Japanese Patent Laid-open Publication No. Heisei 6-269447 can be referred. In this method, various coefficients of the medium being studied, including sonic velocity, are hypothesized, a hypothetical model of the transformation of a propagating waveform is calculated with a theoretical formula using those coefficients, by comparing the calculated waveform with measured waveform improved estimates of the above coefficients, including sonic velocity, are obtained.
As an example of an ultrasonic diagnostic apparatus performing optimum focusing by correcting the sonic velocity, Japanese Patent Laid-open Publication No. Heisei 2-274235 can be referred. In the apparatus of said example, an operator sets the sonic velocity of the medium from the console and then modifies the focus. Also, as an example of an ultrasonic diagnostic apparatus that can perform focusing automatically in a region where the focus is not made on a sectional image, Japanese Patent Laid-open Publication No. Heisei 8-317923 and No. Heisei 10-066694 can be referred. In the apparatus of this example, the human body is regarded as a non-uniform medium and the delay time is controlled in accordance with the living body, which is the object to be examined. According to this method, optimum focusing can be automatically obtained.
In the art disclosed in Japanese Patent Laid-open Publication No. Heisei 6-269447, however, the medium composition has to be hypothesized and also the calculation method is complicated. Moreover, the long time it takes to do this calculation and compare it with the actual measured waveform is a drawback. The art disclosed in Japanese Patent Laid-open Publication No. Heisei 2-274235 does not contain a method of automatically estimating sonic velocity, it puts a great burden on the user, as well as lacking accuracy. Moreover, since the arts shown in Japanese Patent Laid-open Publication No. Heisei 8-317923 and No. Heisei 10-066694 do not attempt the estimation of sonic velocity of the medium, good images cannot be obtained for the whole range of ultrasonic beam scanning object, which is a problem to be solved.